Microelectromechanical Systems (MEMS) sensors are widely used in applications such as automotive, inertial guidance systems, household appliances, protection systems for a variety of devices, and many other industrial, scientific, and engineering systems. Such MEMS sensors are used to sense a physical condition such as acceleration, pressure, angular rotation, or temperature, and to provide an electrical signal representative of the sensed physical condition.
Capacitive-sensing MEMS designs are highly desirable for operation in both acceleration and angular rotation environments and in miniaturized devices, and due to their relatively low cost. Capacitive accelerometers sense a change in electrical capacitance, with respect to acceleration, to vary the output of an energized circuit. One common form of accelerometer is a two layer capacitive transducer having a “teeter-totter” or “see saw” configuration. This commonly utilized transducer type uses a movable element or plate that rotates under z-axis acceleration above a substrate. The accelerometer structure can measure two distinct capacitances to determine differential or relative capacitance.
FIG. 1 shows a top view of a prior art MEMS capacitive accelerometer 20 which is adapted to sense z-axis acceleration. Accelerometer 20 is constructed as a conventional hinged or “teeter-totter” type sensor. Capacitive accelerometer 20 includes a substrate 22 having a generally planar surface. Electrode elements 24 and 26 (shown in dashed line form) are formed on the planar surface of substrate 22. In addition, a suspension anchor 28 is formed on the planar surface of substrate 22. A movable element 30, commonly referred to as a “proof mass,” is flexibly suspended above substrate 22 by one or more rotational flexures, commonly referred to as torsion springs 32, that interconnect movable element 30 with suspension anchor 28. As shown, an opening 34 extends through movable element 30, and suspension anchor 28 is positioned at an approximate center of opening 34 along a rotational axis 36 of movable element 30.
Movable element 30 is adapted for rotation about rotational axis 36 in response to acceleration, thus changing its position relative to the underlying electrode elements 24 and 26. More particularly, torsion springs 32 are subjected to twisting (i.e., shear stress) about their axes coincident with rotational axis 36 in response to z-axis acceleration applied to movable element 30. This change in position results in a set of capacitors whose difference, i.e., a differential capacitance, is indicative of acceleration. Typically, torsion springs 32 are straight bars formed having an appropriate spring constant that allows for rotation of movable element 30 about rotational axis 36 and return to its neutral position.